Petri dish for cultivating bacteria and method of inspecting drug susceptibility

ABSTRACT

A Petri dish for cultivating bacteria used for isolating the bacteria in an enrichment culture for performing drug susceptibility tests, etc. having a mouth covered with at least one sheet and at least one small opening provided on the bottom of the Petri dish, partitions having a height equal to the distance between the bottom of the Petri dish and the sheet may be provided in the Petri dish, and a sealing lid may be fitted over the small openings. A method of inspecting drug susceptibility of bacteria which are isolated from clinical specimens etc. or purely cultured after their isolation using the Petri dish. The method using the fractionized Petri dish having a pervious sheet as the cultivation surface assorts different kinds of media from each other, the media being different in the kind of drug contained in each media or media containing the same drug but in different concentrations in each section of the Petri dish into which the bacteria is inoculated.

This is a division of application Ser. No. 774,757, filed Sept. 11, 1985now U.S. Pat. No. 4,775,628.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Petri dish for cultivating bacteriawhich is used for isolating bacteria, an enrichment culture, drugsusceptibility tests, etc. and a method of inspecting drugsusceptibility of bacteria which are isolated from a clinical specimenetc. or purely cultured after isolation by using the Petri dish.

2. Prior Art

When performing bacterial inspection in a hospital, in order to detectthe pathogen to which the infectious disease is ascribed, it isnecessary to perform operations such as isolation of the culture andenrichment of the bacterial culture from the material to be inspected.Particularly, the results obtained from bacterial inspection are veryimportant since recent trends make it more difficult to identifypathogenic bacteria through clinical diagnosis due to complicatedpathologic configurations of infectious diseases.

However, media suitable for growth of bacteria varies depending upon thekind of bacteria. Thus, when screening pathogenic bacteria, an adequatemedium must be prepared for the growth of each detectable pathogenicbacteria possibly present. Bacteria which can be found in bloodspecimens are: staphylococcus, streptococcus, pneumococcus,enterococcus, haemophylus, salmonella, escherichia coli, pseudomonasaerouginosa, anaerobic bacterium, campylobacter, brucella, etc. In orderto isolate any of these bacteria, it is usually necesary to preparemedia including blood agar medium, chocolate agar medium, isolationmedium for anaerobic bacterium, medium for enrichment, etc. for everyspecimen.

Furthermore, salmonella, dysentry bacillus, pathogenic coli, versiniaenterocolitica, klebsiella oxytoca, vibrio, staphylococcus, bacilluscereus, etc. can be detected from feces and the media needed for any ofthese bacteria include preparations of BTB lactose agar medium, DHL agarmedium, SS agar medium, isolation medium for anaerobic bacterium, mediumfor enrichment, etc.

When producing these media with a conventional Petri dish forcultivating bacteria, it is necessary to heat the media solution for along time because the media contains agar. Also, the Petri dish must beplaced in a horizontal, stationary position in order to facilitatecoagulation of the agar. In addition, almost all conventional Petridishes for cultivating bacteria have one medium prepared per set ofPetri dishes. Thus, the number of Petri dishes required for one specimenis increased and inoculations must be performed one at a time for eachone.

With the above-disadvantages in mind, a method was conceived whereinseveral kinds of media are poured separately into a fractionalized (orsectioned) Petri dish. However, the inside of the dish must be providedwith partitions since the inoculation procedure must be performedsection by section (fraction by fraction). Because the prepared media isdifferent in levels from one section to another, there is no improvementin using a sectioned or fractionalized Petri dish since individualmediums must be prepared for each section and further, since eachsection must be individually inoculated as in the above-described methodusing one Petri dish per medium.

Furthermore, in order to prepare various kinds of plate media using aconventional Petri dish for cultivating bacteria, in addition to thenecessary complicated steps of preparing the solution, sterilization,separate pouring and coagulation of the components of the media; eachoperation must be performed in an aseptic atmosphere as possible.Usually, separate pouring of one medium per Petri dish increases thenumber of Petri dishes necessary for one specimen. Therefore, it isalmost impossible to adequately prepare the necessary amount of mediarequired in a hospital's test room where the number of specimens and thekind of bacteria handled are not always constant. For this reason, manyfacilities make or purchase the estimated amount of media necessary fora predetermined period at one time and use it gradually, storing it in arefrigerator. In this case, since the media when purchased and/or storedis only protected by a fitted lid around the top of the containercontaining the media, there is a problem in that humidity and/or variousbacteria may contaminate the media.

When conventional media is used it must be dried in advance, but mediawhich is in the de-airing state during storage or at the time ofpurchase has oxygen dissolved in the air during such drying process sothat, if anaerobic bacteria are handled, it is necessary to leave themedia in an anaerobic chamber after drying.

Also, when applying chemotherapy to bacterial infectious diseases,selecting and using a drug which is most effective in causing thebacterium such as the disease is recommended. Nevertheless, the kind andconcentration of the drug which will be effective on the bacteria variesdepending on the kind of bacteria, and the recent spread of chemotherapytriggers the appearance of bacterial stock (resistant bacteria) whichshows a tolerance to a druge which has been conventionally effective.Therefore, if it is determined that a particular bacterium is causing aninfectious disease, it is almost impossible to assume an effective drug.Due to the above, for the purpose of putting therapy which is clinicallyadequate and reasonable into practice, it should not be forgotten thatdrug susceptibility inspection is one of the most important procedureswhen inspecting bacterium.

Methods of inspecting the drug susceptibility of bacteria include thedilution method, the diffusion method, and nephelometry. The agar platedilution method used an agar medium and the broth dilution methodbelongs to the dilution method. The sensitive tablet method, thesensitivity disk method, the decantation plate method, and the verticaldiffusion method are diffusion methods. In terms of the actual sitewhere the bacteria is inspected, however, the agar plate dilution methodand sensitivity disk method are used mainly from an operation-orientedperspective.

The sensitivity disk method is used to make a judgment based upon abiogenetic inhibition circle or inhibition zone of the bacteria to betested which occurs in response to diffusion of the drug aftercultivating the bacteria in the media, the bacteria to be tested isinoculated in advance, when the bacteria does not contain the drug andthen a paper disk impregnated with the drug is subsequently placedthereon for a predetermined time. Although this method is widelyutilized because it is easily performed and permits testing a pluralityof drugs and concentration samples with one Petri dish, it causesthickness, inclination, or the kind of medium on which the disk isplaced causes the inhibition circle to flucuate in magnitude so it islikely to lack reproducibility. On the other hand, method using athree-concentration disc, wherein a judgment is made on the basis ofonly the existence of an inhibition circle, not its magnitude, issometimes used. However, since the setup width of the drug concentrationis larger, such a method is not suitable for measuring the minimuminhibitory concentration (MIC) of the bacteria to be tested.

The agar plate dilution method uses media having a predeterminedconcentration of the drug contained therein in advance. This is betterfor reproductibility and can freely set the concentration of the drug.However, since one medium per drug or concentration setup must be usedand inoculation must be performed for every media, making this method iscomplicated.

The purpose of conventional drug susceptibility inspection is to obtaina treating principle regarding the drug concentration at which growth ofall of the bacteria is inhibited. However, it disregards the degree ofdrug susceptibility of the stock of bacteria to be tested correspondingto each drug concentration. In order to obtain more adequate principlesas to the infectious disease, which has a tendency to be complicated, itis necessary to collect further detailed information on the bacteriacausing such disease.

SUMMARY OF THE INVENTION

The object of the present invention is solve the problems inconventional Petri dishes for cultivating bacteria. Specifically, thepresent invention will solve not only the complicated proceduresrequired to prepare the media but also the extremely inefficientinoculation method wherein bacteria is inoculated into one medium one ata time.

It is further an object of the present invention to solve the problem ofthe media getting dried or contaminated by humidity and various bacteriaduring storage and/or while in transit.

It is still a further object of the present invention to solve theproblems such as the complicates procedures required to prepare themedia and inoculation of the above-mentioned conventional inspectingmethod and lower reproducibility of the sensitivity disk method.

In order to accomplish the above object, in the present invention themouth of the upper end of the Petri dish is covered with at least onesheet, the bottom of the Petri dish is provided with at least oneaperture, and, if necessary, partitions of an height equal to thedistance between the bottom of the Petri dish and the above-mentionedsheet, and a sealing lid mounted on the foregoing small aperture.

The sheet may be previous to the component of the media (hereinafterreferred to as the pervious sheet) with which the Petri dish is filledand may be adapted to be able to hold bacteria. It is also possible toadapt the sheet to be a sheet impervious to the component media(hereinafter referred to as the impervious sheet), in which case thesheet needs to be able to detachably cover the Petri dish proper. Theimpervious sheet can be adapted to be impervious also to water andoxygen. It is further acceptable to detachably cover the outside of thepervious sheet with the impervious sheet.

The method of the present invention, using the fractionized Petri dishwith the pervious sheet as the cultivation surface combines differenttypes of media. The media may vary in the type of drug contained in eachmedium or may contain the same type of drug but in differentconcentrations in each fraction (section), into which the bacteria isinoculated, thereby solving the problems of the above-mentionedconventional method.

Thus, preparation of media can be easily and aseptically performed bypouring separately the packing for the medium into the Petri dish fromthe small aperture provided on the bottom of the Petri dish and, whenperforming inoculation operation, unification of the height of thecultivation surface by the sheet enables the entire region of thefractionized medium to be inoculated with bacteria in one step.

When using an impervious sheet, the medium does not get dried, orhumidity and various bacteria do not contaminate the medium duringstorage or transit. Since the medium is sealed so as to not directlycontact air, deterioration due to oxidation is inhibited. Also at thetime of the inoculation operation, when the impervious sheet is removed,a cultivation surface of the same plane can be obtained, even if themmedium is fractionized, thereby inoculating the bacteria into the entirefractionized medium in one step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cut-away exploded perspective view of the firstembodiment of the Petri dish for cultivating bacteria of the presentinvention using a pervious sheet;

FIG. 2 is a bottom view of the lid of the Petri dish of the firstembodiment;

FIG. 3 is a bottom view of the Petri dish of the first embodiment;

FIG. 4 is a partially cut-away exploded perspective view of the secondembodiment of the present invention;

FIG. 5 is a bottom view of the lid of the Petri dish of the secondembodiment;

FIG. 6 is a bottom view of the Petri dish of the second embodiment;

FIG. 7 is a bottom view of the third embodiment of the present inventionusing an impervious sheet;

FIG. 8 is a sectional view taken on the line A--A of FIG. 7;

FIG. 9 is a bottom view of the Petri dish of the fourth embodiment;

FIG. 10 is a sectional view taken on the line B--B of FIG. 9;

FIG. 11 is a partially cut-away exploded perspective view of the fifthembodiment of the present invention;

FIG. 12 is a partially cut-away exploded perspective view of the sixthembodiment of the present invention illustrating an Petri dish usingboth a pervious and impervious sheet;

FIG. 13 is a bottom view of the Petri dish of the sixth embodiment ofthe present invention;

FIG. 14 is a sectional view taken on the line C--C of FIG. 13; and

FIG. 15 is a partially cut-away exploded perspective view of the seventhembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A description of the embodiment according to the present invention willbe made in conjunction with the accompanying drawing.

FIG. 1 to 6 illustrate the embodiment using the perivous sheet.

Numeral 1 is a Petri dish which is made of materials such as plastic orglass and is preferably transparent. The mouth in the upper edge of thePetri dish is covered with a previous sheet 2, and the bottom 3 of thePetri dish is provided with a small aperture 4.

The above mentioned pervious sheet 2 needs to be provided with fineholes or clearances so that the components of the medium filled betweenthe Petri dish 1 and the pervious sheet 2 can be leached out to thesurface of the pervious sheet 2. As to the material of the pervioussheet, a porous film or a porous rubber film or membrane comprisingsynthetic resins such as cellulose ester sytem, polypropylene system,polycarbonate system, polyvinylidene fluoride, polymer belonging toaromatic system, etc. may be preferably used. When the medium fillingthe Petri dish contains a solidfying agent such as agar, etc. materialssuch as cloth, paper, etc., the clearance of which being wider, may beapplicable. If a synthetic resin film which is hydrophobic is used, anadvance hydrophilic treatment must be applied thereto.

As for the strength of the pervious sheet 2, the only requirement isthat it must lie flat when the Petri dish is filled with medium. If thematerial needs any reinforcement, synthetic fiber, etc. can be used.Also, in the case of solid medium, reinforcement can be achieved byinserting a flat reinforcing member into the space between the pervioussheet 2 and the Petri dish lid 5, which will be explained later, untilthe medium is solidified.

Furthermore, the small aperture 4 on the bottom 3 of the Petri dish 1preferably needs to be provided with a sealing lid 6 in a detachablemanner when the medium in the Petri dish contains a solidifying agentsuch as agar so that the inside of the Petri dish 1 which has beensterilized can be kept aseptic. However, such sealng lid is notnecessarily required.

Numeral 5 is a Petri dish lid which is made of plastic or glass and ispreferably transparent. The internal periphery of the lid 5 is formedwith an annular projection 7. This projection 7 forms a gap between thepervious sheet 2 and the Petri dish 5. When the Petri dish lid 5 isfitted onto the Petri dish 1. Thus, a colony of bacteria grown on thesurface of the pervious sheet 2 is prevented from sticking to theinternal surface of the Petri dish lid 5 and can be kept adequatelyair-tight. Incidently, while the projection 7 is provided on theinternal periphery of the Petri dish lid 5, it is not limited to thisplace and it is possible to form the projection 7 on the periphery ofthe pervious sheet 2 or on the periphery on the upper edge of the Petridish 1.

The above-mentioned Petri dish 1 can be provided with partitions 8 onthe bottom. The top of the partitions reach the pervious sheet 2 whichcover the Petri dish 1. In this case, all the sections 9 formed by theparitions 8 must be provided with small aperture 4, respectively. Asshown in FIGS. 4 to 6, for example, when four partitions are provided,small apertures 4 at the center of the bottom 3 of the Petri dish 1 cancommunicate with each of the sections 9. In this case, the sealing lid 6can be used as a plug having a cross slit 10 as shown in FIG. 4.

FIGS. 7 to 11 illustrate an embodiment using an impervious sheet. ThePetri dish 1 is detachably covered with an impervious sheet 11. Theimpervious sheet 11 can be of any thickness but must be impervious towater and oxygen. The material used to make up the impervious sheet ispreferably a synthetic resin membrane or film which is higher in oxygengas barrier properties, such as polyacrylonitrile nylon,polymethacrylonitrile, ethylene vinyl alcohol polymer, polyvinylidenechloride, polyethylene terephtalate, polyvinyl chloride, etc. Othermaterials on which the above listed resins which have higher oxygen gasbarrierr properties or metallic foil such as aluminum foil, etc. islaminated can meet the requirements of the present invention as long asthey fulfill the relevant purpose. The thickness of the impervious sheet11 is subject to the material it is composed of, but meets therequirements only if the strength of the material allows the surface ofthe medium to remain flat when the Petri dish is being filled with themedium.

In the case of a solid medium, a flat reinforcing member is insertedinto the gap between the impervious sheet 11 and the Petri dish lid 5until the medium is solidified to ensure reinforcement thereof.Furthermore, if the small aperture 4 on the bottom 3 of the Petri dishis covered detachably with a sealing lid 6, it is preferable for theinside of the Petri dish, which as already been sterilized, to becomeaseptic. If so, when media is poured separately into the Petri dish,similar to a conventional Petri dish, a completely aseptic Petri dishcan be provided.

As shown in the embodiment of FIGS. 9 and 10, the bottom 3 of the Petridish 1 can be provided with an inclination from the periphery of thePetri dish 1 to the small aperture 4. In this case, when pouring themedia separately into the Petri dish, it is possible to fill the Petridish with media without any foam occurring.

FIG. 11 illustrates a case wherein the partitions 8 divide the Petridish 1 into four sections. According to this embodiment, each section 9is provided with a small aperture 4. In this case, after the media ispoured into each section 9, the small apertures 4 can be sealed withsealing lids 6, respectively. Thus, fear of the bacteria beingcontaminated is advantagesouly minimized.

FIGS. 12 to 15 illustrate an embodiment using both a pervious sheet 2and an impervious sheet 11. The mouth of the Petri dish 1 is coveredwith the pervious sheet 2, which is in turn detachably covered with theimpervious sheet 11.

The Petri dish for cultivating bacteria according to the presentinvention can be utilized for all kinds of agar media except for liquidof semi-fluid media.

Next, a description of an embodiment describing the method of inspectingdrug susceptibility of bacteria according to the present invention willbe given.

In this embodiment the Petri dish shown in FIGS. 4 to 6, i.e.four-section Petri dish wherein the mouth of the Petri dish 1 is coveredwith a pervious sheet 2, is used. The Petri dish 1 is provided withpartitions 8 whose height is equal to the distance between the bottom 3of the Petri dish and the pervious sheet 2. The bottom 3 of the Petridish is provided with small apertures 4 which communicate with all thesections 9 produced by the above-mentioned partitions 8, and the smallapertures 4 are sealed with a sealing lid 6.

Porous poly vinylidene fluoride membrane (bore: 0.45 microns) is used asthe above-mentioned pervious sheet 2. Using Muller-Hinton agar medium asthe medium, a fixed amount of diluted solution of each drug is added tothe medium in order to obtain the drug concentrations shown in Table 1.One section of the Petri dish is left as a control and the media arepoured separately three steps by three steps into three sections 9 andsolidified therein in accordance with the drug concentration. (Since theoperation requires three steps per Petri dish, twelve steps in total arerequired for the operation and four Petri dishes are needed). As thecontrol medium, Muller-Hilton medium is separately poured.

First of all, pus from a patient, as a specimen, is manually inoculatedon the pervious sheet 2 of the above-mentioned four-fraction Petri dish.

Next, the pus is inoculated by direct method, using a spiral plater(made by Spiral System Instruments, U.S.A.). This spiral plater, whichis an inoculating device incorporated into an automatic system formeasuring the number of live bacteria, developed and evaluated by theFDA in the U.S.A., performs the inoculation process such that thematerial to be tested is spirally smeared on the medium, while thematerial to be tested is being given any of its density gradient whichis subject to the distance from the center of the medium.

For this reason, according to the method of the present invention, ifthe material to be tested is smeared on the pervious sheet 2 forinoculation at the predetermined density gradient, measuring theisolated colonies per fixed area is simple to complete and enables thetotal number of live bacteria contained in the unit quanity of materialto be tested and counted out. Furthermore, of course, in addition todrug susceptibility value in accordance with conventional standards,patterns of the sensitivity of the bacteria causing a particular diseasein relation to each drug at its various concentrations stages can beeasily and rapidly obtained.

When using the above-mentioned spiral plater, the pathogenic bacteria inthis specimen, staphylococcus aureus, is isolated at the rate of 2.8×10⁴CFU/ml.

Using conventional judgment standards based on the existance of growth,from Table 1 a judgment can be made that the MIC value of ABPC to theabove-mentioned isolated bacteria is 1.5 μg/ml, that the MIC value ofCTM with respect thereto is 0.78 μg/ml, and the MIC of CZX, CM, and NFLXwith respect thereto are 6.25, over 50, 3.13 μg/ml, respectively.

Paying attention to changes in the number of isolated colonies at thecorresponding concentration of each drug may suggest that according toconventional standards, the MIC value of GM is over 50 μg/ml, butactually, at a concentratin of 6.25 μg/ml about 50% of the bacteriatested shows sensitivity.

                                      TABLE 1                                     __________________________________________________________________________    Pathogenic bacteria: Staphylococcus aureus                                    Controlling medium: Muller-Hinton medium (2.8 × 10.sup.4 CFU/ml)        Drug   Name of drug                                                           concentration                                                                        ABPC   CTM   CZX    GM    NFLX                                         (μg/ml)                                                                           (CFU/ml)                                                                             (CFU/ml)                                                                            (CFU/ml)                                                                             (CFU/ml)                                                                            (CFU/ml)                                     __________________________________________________________________________    0.025  2.7 × 10.sup.4                                                                 2.9 × 10.sup.4                                                                2.8 × 10.sup.4                                                                 2.9 × 10.sup.4                                                                2.8 × 10.sup.4                         0.05   2.8 × 10.sup.4                                                                 2.6 × 10.sup.4                                                                2.9 × 10.sup.4                                                                 2.6 × 10.sup.4                                                                2.8 × 10.sup.4                         0.1    2.9 × 10.sup.4                                                                 2.9 × 10.sup.4                                                                2.7 × 10.sup.4                                                                 2.9 × 10.sup.4                                                                2.6 × 10.sup.4                         0.2    2.6 × 10.sup.4                                                                 2.6 × 10.sup.4                                                                3.0 × 10.sup.4                                                                 2.6 × 10.sup.4                                                                2.9 × 10.sup.4                         0.39   4.3 × 10.sup.3                                                                 1.8 × 10.sup.4                                                                2.7 × 10.sup.4                                                                 2.6 × 10.sup.4                                                                2.8 × 10.sup.4                         0.78   1.2 × 10.sup.2                                                                 0     2.5 × 10.sup.4                                                                 2.9 × 10.sup. 4                                                               2.7 × 10.sup.4                         1.56   0      0     5.8 × 10.sup.3                                                                 2.9 × 10.sup.4                                                                3.5 × 10.sup.3                         3.13   0      0     3.6 × 10.sup.2                                                                 2.6 × 10.sup.4                                                                0                                            6.25   0      0     0      1.7 × 10.sup.4                                                                0                                            12.5   0      0     0      8.0 × 10.sup.3                                                                0                                            25     0      0     0      6.8 × 10.sup.3                                                                0                                            50     0      0     0      6.2 × 10.sup.3                                                                0                                            __________________________________________________________________________     ABPC: ampicillin                                                              CTM: cefotiam                                                                 CZX: ceftizoxime                                                              GM: gentamicin                                                                NFLX: norfloxacin                                                        

According to the present invention, since the Petri dish for cultivatingbacteria has the foregoing construction, filling the Petri dish withmedium does not require that the Petri dish lid 5 be opened so that themedia may be separately poured therein and makes it possible for thePetri dish to be turned reversably while the Petri dish lid 5 is beingfitted. Also, the media are separately poured into the Petri dish fromthe small opening 4 provided on the bottom of the Petri dish. For thisreason, since the Petri dish lid 5 need not be opened when separatelypouring media into the dish, there is less chance of contamination byvarious bacteria in comparison with conventional types of Petri dishes.

Also, when the pervious sheet 2 is used, the medium is solidified on thepervious sheet 2 working as a bottom surface, and the pervious sheet 2acts as the cultivation surface, when the media are provided for bothinoculation and cultivation. Thus, solidification of the medium need notbe performed on a flat table as required conventionally, and vibrationsduring solidification do not cause turbulence on the surface of themedium. When using the impervious sheet 11, if the impervious sheet 11is uncovered when the medium is provided for inoculation andcultivation, the same effect can also be obtained.

When using the sectioned (or fractionalized) Petri dish in accordancewith the present invention, the height of the cultivation surface of themedia with which each fraction is filled is unified by the pervioussheet 2 or the impervious sheet 11. Therefore, inoculation by anautomatic inoculating device such as a spiral plater, which has problemswhen used with conventional fractionalized Petri dishes due todifferences in the levels of the media between the partitions 8, becomespossible. Also, when performing manual inoculation, it is possibleinoculate bacteria in a majority of media with one operation.

Furthermore, when using the Petri dish with the pervious sheet 2, theshape of the medium with which the Petri dish 1 is filled is restrictedby the Petri dish 1 and the pervious sheet 2. Therefore, when resin ofhigher water absorbency, including polyacrylic soda, starchpolyacrylate, PVA, etc., or a solidifying agent including alginic acidorginating from natural substances and carboxy methyl cellulose, issealed in the Petri dish 1, filling with a liquid medium is alsopossible. Thus, preparation of a medium containing a drug whichconventionally requires a very complicated procedure is easily performedin the present invention. In addition, if each of the sections 9 of thePetri dish 1 is filled with media which contain different concentrationsof a particular drug, the Petri dish of the present invention canfurther be used conveniently for drug susceptibility testing.

When the Petri dish with the impervious sheet 11 is used, during storageand transit after the Petri dish is filled with media, the media issealed between the impervious sheet 11 and the Petri dish 1. Thus, themedia does not contact air and drying out and/or deterioration due tooxidation is prevented. Also, the cultivation surface of the media isprevented from getting wet due to humidity and further prevented fromgetting contaminated by various types of bacteria during storage. Sinceit is possible for media poured separately in the Petri dish to bestored in a state screened from air, when cultivating anaerobicbacteria, if media deareated in advance by pressure reduction, boiling,ultrasonic treatment, etc. are separately poured, a medium can beprovided which has a smaller amount of dissolved oxygen in it thanconventional media.

Incidentally, the Petri dish for cultivating bacteria according to thepresent invention can also be used to screen pathogenic bacteria withseveral pieces of dishes by varying adequate media in their respectivesections.

According to the method of the present invention, since utilization ofthe pervious sheet 2 as the cultivation surface enables an extremelyflat cultivation surface to be obtained in spite of the use of afractionalized Petri dish, in addition to making manual bacteriainoculation easier, it is possible to utilize an automatic inoculatingdevice such as a spiral plater.

We claim:
 1. A Petri dish for cultivating bacteria characterized in thata mouth of the upper edge of said Petri dish is covered with a sheet,said sheet being pervious to a component of a media to be filled insidesaid Petri dish and able to hold bacteria on the surface thereof, saidPetri dish being further provided with a small aperture in the bottomand sectioned by partitions, the height of said partitions reaching tosaid sheet from the bottom of said Petri dish.
 2. A Petri dish asdefined in claim 1, wherein said Petri dish is sectioned by partitions,the height of the partitions being equal to the distance between thebottom of said Petri dish and said sheet.
 3. A Petri dish as defined inclaim 2, wherein said small aperature is detachably mounted providedwith a sealing lid.
 4. A Petri dish as defined in claim 1 wherein saidsmall aperture is detachably provided with a sealing lid.
 5. A Petridish as defined in claim 1, wherein said small aperture is detachablyprovided with a sealing lid.
 6. A Petri dish as defined in claim 1,wherein said small aperture is detachably provided with a sealing lid.